1. Field of the Invention
The present invention relates to a process for removal of transition metals from polymers.
2. Description of the Background
Purification of polymers is known per se. By way of example, low-molecular-weight organic constituents can be removed from mixtures by extraction processes, by bringing solid polymers into contact with extractants. WO 02/28916 describes, by way of example, removal of oligomers from ethylene-acrylate copolymers. These low-molecular-weight organic by-products are produced during the synthesis and can in particular cause odor impairment and taste impairment in packaging for food or drink.
However, the proportion of these constituents present is relatively high, for example from 0.2 to 3.7% by weight, and this removal process is not quantitative, but gives different results, depending on the extractant used. These results make it obvious that at least 0.04% by weight of low-molecular-weight compounds remain in the polymer even after an extraction process. In comparison with this, the amount of the transition metal complex present in the polymer is in the range of about 400-500 ppm.
For this reason, it should be obvious that a very small proportion of this type could hardly be reduced by an extraction process described in WO 02/28916.
Accordingly, no inorganic metal compound is mentioned in WO 02/28916. The same applies to the specification of application EP 1 058 567, in which extraction of polymer granules is described for removal of oligomers and monomers which are produced during condensation of nylon-6.
Other processes are known for removal of transition metals from polymers. By way of example, a copper catalyst has been used during synthesis of polyphenylene oxide, and is removed after the polymerization process via aqueous liquid-liquid extraction of the solution (cf. Ullmanns Encyclopedia of Industrial Chemistry, 1992 5th edition, vol. 26 a, 606 et seq.) However, a disadvantage of this method is that many polar polymers act as a suspension stabilizer and inhibit separation of the two liquid phases. These processes cannot therefore be used for purification of polymethyl methacrylate, for example.
The process known as ATRP (atom transfer radical polymerization) is an important process for preparation of a wide variety of polymers, such as PMMA and polystyrene, with relatively good control of their structure, the molecular weight, and the molecular weight distribution. These processes use transition metals, such as copper, for controlled polymerization of vinyl compounds. This process permits the construction of very particular polymers which are very difficult or impossible to prepare by traditional polymerization processes. A disadvantage of this process is the use of transition metals which have to be removed from the polymerization mixture after the polymerization process.
Various methods have been proposed for this. On the laboratory scale, the method most often used for removal of the catalyst, such as copper, is adsorption on aluminum oxide followed by precipitation of the polymer with polar precipitants, such as methanol. This type of process is industrially disadvantageous for various reasons.
Firstly, the form in which the polymer is present after the precipitation process is non-uniform, for example granules, and this makes removal of precipitants and work-up difficult. Furthermore, the precipitation process produces large amounts of the precipitant mixed with the solvents and with other constituents to be removed, such as residual monomer, and complicated separation of these is required.
There are also known processes in which the solid catalyst is removed from the liquid polymer-containing solution. Here, the catalyst itself becomes insoluble, for example via oxidation, or is bound, before or after the polymerization process, to a solid adsorbent or to a swollen but insoluble resin. The liquid polymer-containing phase is separated from insoluble material via filtration or centrifuging. By way of example, CN 1210111 describes a process where an adsorbent (in particular activated charcoal or aluminum oxide) is added to the polymer solution after the ATRP process and then is removed via filtration. A disadvantage here is that complete removal can be achieved only via very large amounts of adsorbent, because the content of transition metals in the reaction mixture is relatively small. Furthermore, these adsorbents are relatively expensive and require complicated renewal processes.
DE 100 15 583 describes another ATRP process which uses a non-polar solvent for the polymerization process. The transition metal catalyst becomes insoluble via oxidation during or after the reaction, and can be removed by filtration. These processes are generally suitable only for preparation of relatively non-polar polymers. If polar polymers are prepared, polymethyl methacrylate for example, these polymers are then insoluble in the solvent. This process can therefore only be used for very specific polymers.
Quirk and collaborators describe a further process for the removal of transition metal catalysts (cf. Liou, S.; Malaba, D.; Brittain, W.; Lee, Y.; Quirk, R. Poly. Prep. (Am. Chem. Soc., Div. Poly. Chem.) 1999, 40(2), 380). This process uses an ATRP catalyst with a specific ligand, the catalyst complex therefore being soluble during the polymerization process, but, in contrast, becoming insoluble under the conditions of purification, thus permitting removal of the transition metal catalyst. A disadvantage of this process is the use of very specific ligands, which are firstly expensive and secondly are also not capable of general use.
EP 1 132410 describes a two-stage purification process. In a first step, the insoluble portion of the catalyst is removed via centrifuging. In a subsequent step, residual transition metal is then bound via an ion exchanger resin, which is removed by filtration. A disadvantage here again is the use of specific adsorbents, which are relatively expensive when used on an industrial scale.
There are also known methods which carry out the polymerization process with a catalyst previously immobilized on a solid or on a gel (cf. WO 00/56795, WO 01/062803, Brittain and collaborators, Polymer Prepr. (Am. Chem. Society., Div. Poly. Chem.) 2002, 43(2), 275). A particular disadvantage of this method is the high costs generated by the catalyst immobilized on a solid. Furthermore, reactions of this type are relatively slow, because the ends of the chains have difficulty in reaching the catalyst center.
A feature of the abovementioned processes is that a liquid polymer solution is removed from the catalyst present in insoluble form. However, removal of undivided or swollen particles from a viscous solution is technically complicated and associated with many disadvantages. By way of example, high rotation rates are needed during centrifuging, and the required separation time here is long. During filtration the filter often blocks, and high pressure has to be exerted here because the viscosity is high. Industrial-scale conduct of these processes is therefore possible only at very high cost.
Furthermore, WO 01/84424 describes a process which binds the initiator to a solid support. The polymerization process produces polymer chains pendant from these solid supports, the chains being separated by cleavage after removal of the catalyst solution. The main disadvantage is the large number of uneconomic steps in the process, in addition to the actual polymerization process. Furthermore, this process can succeed only with filtration and precipitation, which are attended by the abovementioned disadvantages.